In digital electronic systems, it is frequently desirable to generate a digital logic signal that is delayed by an accurate, predetermined amount of time with respect to the trasition of another logic signal. In many of these applications, it is important that the time delay be essentially independent of power supply voltage variations and of ambient temperature variations. (As used herein, the term "essentially" means closely approximating to a degree sufficient for practical purposes.) Circuits utilized to generate such delays include multi-stage hybrid delay lines which typically use small discrete or thick-film inductors and capacitors as the primary elements for establishing the delay of each delay stage. Other circuits utilized for generating accurate delays include monostable multivibrator integrated circuits (also known as "one shots") which typically use external resistors and capacitors for establishing the desired delay.
By comparison to monolithic integrated circuits, though, circuits which require thick film or discrete components tend to be more expensive to manufacture and not as reliable.
Delay circuits have heretofore been implemented in monolithic integrated circuits. Two such integrated circuits are the Dallas Semiconductor Corporation's DS1000 and DS1002. The time delay of each stage in these circuits is primarily determined by the time required to discharge a capacitive element through an MOS field effect transistor. The rate of discharge is controlled by a bias voltage applied to the gate terminal of the field effect transistor. The bias voltage is varied with temperature in a manner to effectively compensate for temperature-induced variations in the field effect transistor. To minimize sensitivity of these monolithic integrated circuits to variations in power supply voltage, circuitry is included therein for producing an on-chip reference voltage which is highly stable with power supply voltage and temperature variations. Such circuitry, however, requires a significant amount of chip area.
In accordance with the foregoing, a need exists for a simple delay stage which can conveniently be fabricated in a monolithic integrated circuit and which is essentially insensitive to variations in the power supply voltage and ambient temperature.